A writing mode describes the directionality of a script, i.e. it describes
the direction the script is to be read. In (X)HTML, the language of a
document is defined by the lang or xml:lang
attributes. To control the rendering of the text, a couple of CSS properties
can be used: the direction property defines
the inline direction of the script, that is left-to-right (like e.g. Latin or
Indic scripts) or right-to-left (like e.g. Arabic and Hebrew scripts).
The writing-mode property, on the other
hand, describes the block direction of the script, namely whether the text
should be layed out horizontally, top-to-bottom (like e.g. Latin or Arabic
scipts), or vertically, right-to-left (like e.g. Chinese scripts). The
default value is horizontal-tb, which means horizontal,
top-to-bottom.
Changing the writing mode of a document, that is, the inline or block direction, not only changes the direction of the script, but also affects several other aspects of the printed document.
The page selector pseudo-classes :recto and :verso
(see Selecting pages) are relative to the direction of
the script. In a left-to-right script, :recto is the right-hand
side of a spread, and :verso is the left-hand side, while in a
right-to-left script these values are inverted: :recto defines
the left-hand side of a spread, and :verso defines the
right-hand side.
Columns change their orientation when the writing mode is changed -
writing-mode vertical-rl arranges the
columns horizontally, top-to-bottom.
(Note that this can be used to rotate content - see Printing wide content sideways and Rotating content in table cells).